they will still gain the benifits of termie armour! (so +1A and the nifty 5+ invulnerable save!)
you simply can't give the squad any kind of weapon upgrade, aside from the unit's aspiring champion who can take stuff from the armoury as normal.
(provided of corse there's a '*' next to the item...)
chosen of tzeentch are generally a waste of pts unless you're playing a very large game where sinking 400+pts into one unit won't cripple your list!
cheers!